PHYS Ashutosh Rath - Homi Bhabha National Institute

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Introduction 4 metals, in the case of metal nanoparticles the conduction band is absent being replaced by discrete states at the band edge because of the quantum-confinement of the electrons. As the spectrum of the potential applications of metallic nanoparticles is diversifying every day, there is a tremendous interest in preparation of metal nanoparticles with uniforms sizes and shapes by cost-effective and easily scalable procedures. Gold has been for many years a case metal to study the changes in physical and chemical properties at nano scale. This precious element has long been known as being essentially chemically inert (and therefore catalytically inactive) in its bulk form [38]. Quite recently, however, it was found that, when dispersed in the form of nanoscale clusters and supported on transition metal oxide surfaces such as TiO 2 [39, 40], Au exhibits a very high (low temperature) catalytic activity for the partial oxidation of hydrocarbons, hydrogenation of unsaturated hydrocarbons and reduction of nitrogen oxides [41]. It was shown that the catalytic properties of Au nanoparticles depend specifically on the support, the preparation method and critically, on the size of the Au clusters, which are active when their diameter is smaller than 3.5 nm [42, 39]. Furthermore, the catalytic behavior depends on weather the overlayer nanostructures are in direct contact with its support material or they will have to overcome a barrier. For example inter diffusion behavior of metal (overlayer) − semiconductor (substrate) system is strongly influenced by the presence or absence of an oxide layer at the interface. It has been shown that the interfacial diffusion of gold and silicon occurs at lower temperature in absence of interfacial oxides than in presence of oxide [28, 29, 43]. It is well known that, at elevated temperature and high vacuum conditions, silicon oxide thin films decompose to expose the clean Si surface [44, 45]. This would contribute to play important role in controlling the inter-diffusion and reaction of gold with silicon. Detailed explanation of thermal decomposition of oxide layer and the resulting growth of gold silicide nanostructures would be presented in this thesis work. Furthermore, gold has been widely used as a catalyst for metal-catalyzed vapor-liquidsolid (VLS) growth [46]. In this method, a nanometer-sized metallic seed particle which forms a low temperature eutectic alloy with the wire material acts as a site of preferential, unidirectional crystal growth. While wire growth is a nonequilibrium process, the equilibrium phase diagram provides important guidance on the conditions such as temperature and liquid composition needed for nanowire fabrication. Unfortunately, these temperatures and compositions cannot be determined from the
Introduction 5 bulk phase diagram, even if the wire is growing very close to equilibrium, due to the major influence of capillarity and stress in these nanoscale objects. Hourlier et al have studied thermodynamically the relative stability of different systems solid and liquid at equilibrium involved in the growth of semiconductor nanowires. They calculated it for the two binary systems Au-Si and Au-Ge [47]. Sutter and Sutter have measured the equilibrium composition of a Au-Ge liquid alloy drop on the end of a Ge nanowire using in situ heating in a transmission electron microscope (TEM) [48]. These results indicate that phase boundaries in the nanowire system are shifted to lower temperatures compared to those of a bulk system. A direct observation of the VLS growth of Ge nanowires was reported by Wu and Yang [49], who identified the various growth stages in correlation to the Au-Ge binary phase diagram. Similar in situ observation of VLS growth of Si NWs by using ultrahigh-vacuum transmission electron microscopy was reported by Ross et al [50]. Haraguchi et al applied the VLS mechanism using gold droplets as catalysts for the growth of III-V NWs, for example, GaAs and InGaAs [51]. In this thesis work, goldsilicide is used as catalyst for the growth of Au-Ge bilobed structures. It is similar to the VLS growth of Ge with Au-Si as transport medium, except molecular-beam source is used instead of a chemical compound vapor like CVD growth. The detail mechanism would be discussed in this thesis. Direct observation of nano structural evolution under dynamic reaction conditions is a powerful scientific tool in materials science. Numerous advancements have been achieved in electron microscopy in the past decades. Not only has the spatial resolution been improved, image recording techniques have been revolutionized by the application of CCD cameras and analytical spectroscopies with almost atomic resolution have been integrated into the standard electron microscope. For example, in-situ electron microscopy provides dynamic information on processes under controlled reaction conditions which can not be obtained directly by other techniques. Some of the earlier in situ experiments carried out in high vacuum in the TEM have produced important results. In-situ experimentation requires specially designed specimen stages that fit into the narrow gap of the objective pole pieces for high resolution microscopy. Nowadays, varieties of specimen stages are available, for example for heating, cooling, electrical probing, straining or indentation of the specimen [52]. Incorporation of a genuine UHV range suitable for surface science studies was started by Pashley [53]
Page 1: DYNAMIC AND STATIC TEM STUDIES ON T
Page 4 and 5: Declaration I, Ashutosh Rath, hereb
Page 6 and 7: a special word of appreciation for
Page 8 and 9: List of Publications 1. 1‡ Epitax
Page 10 and 11: Muller,M. Schowalter, R.Imlau, A. R
Page 12 and 13: Synopsis The thesis work involves a
Page 14 and 15: gold nanostructures on Si (100) sur
Page 16 and 17: Au-Ge bilobed structures were forme
Page 18 and 19: Contents Statement By Author Declar
Page 20 and 21: 8 Summary 107 A Table to summarize
Page 22 and 23: 3.1 As deposited MBE sample showing
Page 24 and 25: 4.11 (a) SEM image taken at 54 o ti
Page 26 and 27: 6.7 Schematic diagram showing the A
Page 28 and 29: Introduction 2 giant magneto-resist
Page 32 and 33: Introduction 6 and has been develop
Page 34 and 35: Experimental techniques 8 2.2 Thin
Page 36 and 37: Experimental techniques 10 for mono
Page 38 and 39: Experimental techniques 12 Layer pl
Page 40 and 41: Experimental techniques 14 signific
Page 42 and 43: Experimental techniques 16 Instrume
Page 44 and 45: Experimental techniques 18 Figure 2
Page 46 and 47: Experimental techniques 20 spherica
Page 52 and 53: Experimental techniques 26 a dynami
Page 54 and 55: Experimental techniques 28 that are
Page 56 and 57: Experimental techniques 30 componen
Page 60 and 61: Experimental techniques 34 electron
Page 64 and 65: Chapter 3 Temperature-dependent ele
Page 66 and 67: Temperature-dependent electron micr
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Chapter 4 Growth of Oriented Au Nan
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Growth of Oriented Au Nanostructure
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Structural modification of gold sil
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Nano scale phase separation in Au-G
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Gold assisted molecular beam epitax
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Summary 108 square /rectangular sha
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Summary 110 the annealed sample whi
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Bibliography [1] M. S. Gudiksen, L.
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BIBLIOGRAPHY 114 [32] Y. He, X. Yu
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BIBLIOGRAPHY 116 [67] M. Ohring Mat
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BIBLIOGRAPHY 118 [97] W. K. Chu, J.
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BIBLIOGRAPHY 120 [131] C. Suryanara
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BIBLIOGRAPHY 122 [167] C. R. Henry,
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BIBLIOGRAPHY 124 [201] E. Sutter an
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PHYS Ashutosh Rath - Homi Bhabha National Institute

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